Catalytic cracking is a commercial process applied by the petroleum refining industry on a very large scale. A large percentage of the refinery gasoline blending pool in the United States is produced using fluid catalytic cracking (FCC) processes. In this process, heavy hydrocarbon fractions (feedstock) are converted into lighter, more desired products by reactions taking place in the presence of a catalyst. The majority of this conversion occurs in the vapor phase within the fluid catalytic cracking unit (FCCU) which contains particles of the catalyst in a moving or ebullating state. The feedstock is, thereby, converted into gasoline, distillates and other liquid products as well as lighter gaseous products of four or less carbon atoms per molecule.
The three characteristic process zones of the FCC process are composed of:    i) a cracking step in which the hydrocarbon feedstock is endothermically converted into the desired lighter products. This occurs by contacting hot, active particles of catalyst, normally without added hydrogen, at pressures of up to about 50 psig and elevated temperatures of up to about 650° C. The catalyst must be of sufficient particle size and strength as to not provide excessive fines which can contaminate the product stream and/or the atmosphere. At the same time, the catalyst particles must be small enough to be capable of being in a fluidized state;    ii) a stripping step in which hydrocarbons adsorbed on the catalyst are removed; and    iii) a regeneration step in which the undersirable carbonaceous residue (coke) remaining on the catalyst is removed by subjecting the catalyst to sufficiently high temperatures to burn off coke from the catalyst. The hot regenerated catalyst is then reused in the cracking step to treat additional feed and to provide the elevated temperature maintained in the cracking zone.
The cyclical processes of cracking and regeneration steps, wherein the catalyst is subjected to high flow rates and temperatures, have a tendency to physically break down the catalyst into smaller particle sizes called fines. These fines detract from the catalysts activity and processability. Thus, as part of an FCC process, the fines are removed from the FCCU in a continuous manner.
Cracking catalysts used in FCC processes are porous powders composed of oxides of silica and alumina. When aerated with gas, the powder attains a fluid-like state that permits its circulation through the various FCC process zones. The major active components of FCC catalysts of concern here are zeolites. The term “zeolite”, as used herein and in the appended claims, refers to synthetic and natural faujasites.
Natural and synthetic zeolites can not be directly used in FCC units as they are of ultra-small particle size which rapidly produce a large amount of fines of 0-20 micron diameter. As stated above, the presence of large amounts of such fines can not be tolerated as they will pollute the atmosphere or require high capacity cyclones and electrostatic precipitators to prevent them from becoming airborne. Further, those skilled in the art appreciate the concept that excessive generation of catalyst fines causes increased addition of fresh particles of desired size and dilution of catalytically viable particle. Thus, the process will reflect increased cost due to attrition of catalyst as well as increased cost in attaining proper removal of fines and/or lower activity of the catalyst composition.
More specifically, natural and synthetic zeolites are powders that have an average particle size of about 20 to about 40 microns and, therefore, are not deemed useful as an FCC catalyst. The specific particle size distribution of formed zeolite may be regulated by controlling the initial spray drying of the formed catalyst. However, in almost all instances the physical integrity (attrition resistance) of the material is poor and, therefore is easily converted into fines within the FCCU.
In view of the above, formed zeolite is conventionally agglomerated into particles having an average diameter of from about 50 to about 150 microns, with from about 60 to 100 microns being preferred. The agglomerated particles deemed useful for FCC processes are formed from a mixture of a zeolite along with additional active matrix material to enhance the particle's conversion activity as well as inactive material and a binder. These components are conventionally combined and formed into particulate material of the desired particle size. Typically, FCC catalysts are composed of from 20 to 40 parts by weight of a zeolite, 0 to 30 parts by weight of an active matrix material, 20 to 50 parts by weight of an inactive matrix material and from 10 to 25 parts by weight of a binder.
Because of the constraint of the particle size, conventional catalyst particles provide only a limited room for those components (e.g., zeolite and alumina) that provide the majority of the catalyst's cracking activity or kinetic catalyst activity. It has been believed that the inclusion of large amounts of inactive matrix materials (e.g. clays) are required to provide a suitable particulate material for use in an FCCU.
In cracking of hydrocarbons, it is desired to maximize output of desirable products while minimizing costs, including those attributable to catalyst handling. The scale of cracking is such that even what appears to be modest improvement in some property may have a large effect on a refinery's profitability.
While modern cracking catalysts have made significant strides to improve their catalytic performance and their physical properties, there is still the need to provide catalysts which can exhibit enhances kinetic catalyst activity while having good physical properties which make them resistant to attrition and production of fines.
It would be highly desired to provide an FCC catalyst that provides high kinetic conversion activity and are hard particulate material resistant to formation of fines.